1. Field of the Invention
The present invention relates to an axial-flow turbine and, particularly, to a gas turbine in which the pressure between a turbine and a diffuser is locally increased so that the thermal efficiency is increased.
2. Description of the Related Art
In general, it has been required that the temperature in a turbine entrance and pressure ratio are further increased to improve the thermal efficiency of an axial-flow turbine, e.g. gas turbine.
Japanese Unexamined Patent Publications (Kokai) No. 5-321896 and No. 11-148497 disclose a solution in which the shape of the front side or the back side of a blade is modified so that the pressure loss caused by shock waves is decreased. In Kokai No. 5-321896, a blade, for example, a rotor blade in which the shape of the front side or the back side thereof is modified, is disclosed. In Kokai No. 11-148497, a blade, for example, a rotor blade in which the maximum thickness portion of the blade is changed from a position of 40% of a chord length to a position of 60% of the chord length, is disclosed.
However, in the above-described two related arts, only a part of the shape of a blade and, especially, only the shape of the front side or the back side of the blade is taken into account, and the shape of the tip portion of the blade is not taken into account. In general, a space between the tip portion of a blade, especially, a rotor blade and the inner wall of an axial-flow turbine passage e.g. a gas turbine passage, substantially does not exist, and they are located in contact with each other. Therefore, in order to further reduce the pressure loss caused by shock waves to increase the efficiency, not only the shape of the front side or the back side of the blade but also the shape of the tip portion of the blade and the inner wall of the axial-flow turbine passage adjacent to the tip portion should be taken into account.
Accordingly, the object of the present invention is to further reduce the pressure loss, caused by shock waves in the vicinity of a tip portion trailing edge of terminal stage rotor blades, so as to improve the efficiency of the axial-flow turbine by modifying the shape of the tip portion of the blades and the shape of the axial-flow turbine passage e.g. the gas turbine passage.
According to an embodiment of the present invention, there is provided an axial-flow turbine comprising an exhaust chamber; a turbine including multiple stage rotor blades, said multiple stage rotor blades including terminal stage rotor blades; an annular diffuser located between the turbine and the exhaust chamber; and an annular axial-flow turbine passage defined by the turbine, the diffuser and the exhaust chamber, wherein fluid flows through the axial-flow turbine passage toward the exhaust chamber, and an annular stepped portion which inwardly projects in a radial direction is formed on the portion of an inner wall of the axial-flow turbine passage that is located on the downstream side of a trailing edge of a tip portion of the terminal stage rotor blades provided in the flow direction of the fluid.
In other words, according to the embodiment of the present invention, the streamline of a fluid passing through the axial-flow turbine passage is inwardly curved between the tip portion trailing edge and the upstream end portion of the stepped portion so that variations in the streamline occurs. Therefore, the pressure is increased to reduce the Mach number, and the pressure loss is decreased to improve the turbine efficiency. Additionally, the Mach number is decreased to reduce the occurrence of shock waves and, thus, damage to the tip portion of the rotor blade can be prevented.
These and other objects, features and advantages of the present invention will be more apparent in light of the detailed description of exemplary embodiments thereof as illustrated by the drawings.